Ionic liquids are liquids that are composed entirely of ions. The so-called “low temperature” ionic liquids are generally organic salts with melting points under 100 degrees C., or often even lower than room temperature. Ionic liquids may be suitable, for example, for use as a catalyst and as a solvent in alkylation.
One class of ionic liquids is fused salt compositions, which are molten at low temperature and are useful as catalysts, solvents, and electrolytes. Such compositions are mixtures of components which are liquid at temperatures below the individual melting points of the components.
Ionic liquids can be defined as liquids whose make-up is entirely comprised of ions as a combination of cations and anions. The most common ionic liquids are those prepared from organic-based cations and inorganic or organic anions. The most common organic cations are ammonium cations, but phosphonium and sulphonium cations are also frequently used. Ionic liquids of pyridinium and imidazolium are perhaps the most commonly used cations. Anions include, but are not limited to, BF4—, PF6—, haloaluminates such as Al2Cl7— and Al2Br7—, [(CF3SO2)2N]—, alkyl sulphates (RSO3—), carboxylates (RCO2—) and many others. The most catalytically interesting ionic liquids are those derived from ammonium halides and Lewis acids (such as AlCl3, TiCl4, SnCl4, FeCl3 . . . etc.). Chloroaluminate ionic liquids are perhaps the most commonly used ionic liquid catalyst systems.
Examples of such low temperature ionic liquids or molten fused salts are the choloraluminate salts. Alkyl imidazolium or pyridinium salts, for example, can be mixed with aluminum trichloride (AlCl3) to form the fused chloroaluminate salts. The use of the fused salts of 1-alkylpyridinium chloride and aluminum trichloride as electrolytes is discussed in U.S. Pat. No. 4,122,245. Other patents which discuss the use of fused salts from aluminum trichloride and alkylimidazolium halides as electrolytes are U.S. Pat. Nos. 4,463,071 and 4,463,072.
U.S. Pat. No. 5,104,840 describes ionic liquids which comprise at least one alkylaluminum dihalide and at least one quaternary ammonium halide and/or at least one quaternary ammonium phosphonium halide; and their uses as solvents in catalytic reactions.
U.S. Pat. No. 6,096,680 describes liquid clathrate compositions useful as reusable aluminum catalysts in Friedel-Crafts reactions. In one embodiment, the liquid clathrate composition is formed from constituents comprising (i) at least one aluminum trihalide, (ii) at least one salt selected from alkali metal halide, alkaline earth metal halide, alkali metal pseudohalide, quaternary ammonium salt, quaternary phosphonium salt, or ternary sulfonium salt, or a mixture of any two or more of the foregoing, and (iii) at least one aromatic hydrocarbon compound.
Aluminum-containing catalysts are among the most common Lewis acid catalysts employed in Friedel-Crafts reactions. Friedel-Crafts reactions are reactions which fall within the broader category of electrophylic substitution reactions including alkylations.
Other examples of ionic liquids and their methods of preparation may also be found in U.S. Pat. Nos. 5,731,101; 6,797,853 and in U.S. Patent Application Publications 2004/0077914 and 2004/0133056.
As a result of use, ionic liquid catalysts become deactivated, i.e. lose activity, and may eventually need to be replaced. Alkylation processes utilizing an ionic liquid catalyst form by-products known as conjunct polymers. These conjunct polymers deactivate the ionic liquid catalyst by forming complexes with the ionic liquid catalyst. Conjunct polymers are highly unsaturated molecules and may complex the Lewis acid portion of the ionic liquid catalyst via their double bonds network system. As the aluminum trichloride becomes complexed with conjunct polymers the activity of the ionic liquid catalysts becomes impaired or at least compromised. Conjunct polymers may also become chlorinated and through their chloro groups may interact with aluminum trichloride and therefore reduce the overall activity of the catalyst or lessen its effectiveness as a catalyst for the intended purpose such as alkylation. Deactivation of the ionic liquid catalyst by conjunct polymers is not only problematic for the alkylation chemistry, but also weighs in heavily on the economics of using ionic liquids because they are expensive catalytic systems and their frequent replacement will be costly. Therefore, commercial exploitation of ionic liquid catalysts during alkylation is impossible unless they are efficiently regenerated and recycled.
Only a few methods for removing conjunct polymers from acidic ionic liquid catalysts in order to regenerate the catalysts have been devised. These methods are described in U.S. Patent Application Publication 2007/0142213 and include hydrogenation, addition of a basic reagent, and alkylation.
The first method, hydrogenation, saturates the double bonds of the conjunct polymers such that they release the acidic ionic liquid catalysts. For hydrogenation to occur, hydrogen must either be fed to the acidic ionic liquid catalyst/conjunct polymer complexes or hydrogen must be produced in situ. This may be done by treating the catalyst containing the conjunct polymers with a metal in the presence of a Bronsted acid where interaction between the metal and the acid produces the needed hydrogen. For example, reacting aluminum metal with hydrochloric acid will produce hydrogen and aluminum trichloride. Treating the spent catalyst containing conjunct polymers with Al metal in the presence of enough HCl will produce the hydrogen needed to saturate the double bonds of the conjunct polymers. After hydrogenation, the hydrogenated conjunct polymers are removed by solvent extraction or decantation and the regenerated ionic liquid catalyst is recovered.
The second method, addition of a basic agent (e.g. amines or ammonium chloride), similarly breaks up the acidic ionic liquid catalyst/conjunct polymer complexes as the basic agent forms new complexes with the catalyst. The basic agent must be carefully chosen so that it is part of the catalyst system undergoing regeneration. Otherwise, the basic agent will simply deactivate the catalyst in the same manner as the conjunct polymers. Additionally, the basic agent will react not only with the acidic ionic liquid catalyst/conjunct polymer complexes (e.g. AlCl3/conjunct polymer complexes) but also with any unbound cation (e.g. AlCl3). Therefore, the basic agent must correspond to the basic parent species of cation from which the ionic liquid to be regenerated was originally produced and the basic agent must be added in an amount sufficient to react with both cations bound in the acidic ionic liquid/conjunct polymer complexes and unbound cations. Then the free conjunct polymers are removed and the remaining new complexes are contacted with additional unbound cations (e.g. AlCl3) to fully regenerate the catalyst. As an example, a used chloroaluminate ionic liquid may be contacted with butylpyridinium chloride to provide butylpyridinium tetrachloroaluminate and free the conjunct polymers and then the butylpyridinium tetrachloroaluminate may be contacted with AlCl3 to fully restore the catalyst's activity.
However, while effective, each of these methods suffers from certain shortcomings. Thus, to take advantage of the potential of ionic liquids as catalysts, particularly in alkylation reactions, the industry continues to search for an effective and efficient ionic liquid catalyst regeneration process.